The present invention relates to ink jet printers and methods of printing by ink jets. The present invention is particularly useful in the apparatus and methods described in our prior U.S. Pat. Nos. 5,969,733, 6,003,980 and 6,106,107, the contents of which are hereby incorporated by reference. The invention is therefore described below with regard to such apparatus and methods, but it will be appreciated that the invention could also be used in other apparatus and methods.
Ink jet printers are based on forming drops of liquid ink and selectively depositing the ink drops on a substrate. The known ink jet printers generally fall into two categories: drop-on-demand printers, and continuous-jet printers.
Drop-on-demand printers selectively form and deposit the ink jet drops on the substrate as and when demanded by a control signal from an external data source. Such systems typically use nozzles having relatively large openings, ranging from 30 to 100 μm.
Continuous-jet printers, on the other hand, are stimulated by a perturbation device, such as a piezoelectric transducer, to form the ink drops from a continuous ink jet filament at a rate determined by the perturbation device. The drops are selectively charged and deflected to direct them onto the substrate according to the desired pattern to be printed.
Continuous-jet printers are divided into two types of systems: binary, and multi-level. In binary systems, the drops are either charged or uncharged and, accordingly, either reach or do not reach the substrate at a single predetermined position. In multi-level systems, the drops can receive a large number of charge levels and, accordingly, can generate a large number of print positions.
The process of drop formation depends on many factors associated with the ink rhelogy (e.g. viscosity, surface tension), the ink flow conditions (e.g. jet diameter, jet velocity), and the characteristics of the perturbation (e.g. frequency and amplitude of the excitation). Typically, drop formation is a fast process, occurring in the time frame of a few microseconds. However, because of possible variations in one or more of the several factors determining the drop formations, variations are possible in the exact timing of the drop break-off. These timing variations can cause incorrect charging of drops if the electrical field responsible for drop charging is turned-on, turned-off, or changed to a new level, during the drop break-off itself. Therefore it is necessary to keep the data pulse precisely in-phase relative to the drop break-off timing, in order to obtain accurate drop charging and printing.
Another type of commonly-occurring printing error is incorrect velocity of the ink drops such that the ink drop is not deflected to its proper position on the substrate. Drop velocity (or jet speed) errors may be produced by many different factors, such as those associated with the ink rhelogy and/or the ink flow conditions. Such errors may be corrected by changing the drop charging voltage applied to the ink drops since the amount of deflection experienced by the ink drops before impinging the substrate depends on the drop velocity, the voltage applied to the deflector plates electric field, and the drop charge.
A still further problem in ink jet printing is the formation of satellites in the stream of drops. Satellites are characterized by volumes which are much smaller (typically by more than one order of magnitude) than the basic drop volume, i.e. the volume within the drop desired to be printed. In the usual capacitively charged configurations, satellites carry a charge similar to the charge carried by the basic drop. The acceleration experienced by charged drops in an electrical field is inversely proportional to their masses. Since the mass of the satellite is much smaller than the mass of the basic drop, satellites will experience a much stronger acceleration inside the deflection field, and may therefore impinge against the deflecting plates. This could result in an electrical breakdown condition or other malfunction of the printer.
The above-cited U.S. Pat. No. 6,003,980 discloses a method and apparatus for sensing improper operation of an ink jet printer by printing test marks on a test strip, and then analyzing the printed test marks. However, such a technique is not always practical or convenient particularly with respect to ink jet printers including a large number of nozzles. In addition, relying on an analysis of printed marks on a substrate for sensing improper operation of an ink jet printer may suffer from lack of consistency because of inconsistencies in the substrates themselves.